Impedance transformers are much used in electrical apparatus, especially electronic apparatus intended to operate in the radio and high frequency range (i.e., 10 to 10,000 MHz or higher). While conventional wire wound step-up or step-down transformers are useful at the lower end of this frequency range, they become ineffective as frequency increases At the upper end of the range, waveguides elements are used to provide impedance transformation. At intermediate frequencies it is known to use transmission line segments for providing impedance transformation.
A variety of transmission line and waveguide elements useful for impedance transformation are described by U. Barabas in "On an Ultrabroad-Band Hybrid Tee", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-27, No. 1, January 1979, pages 58-64 (hereafter Barabas). In particular, Barabas describes a broad-band line transformer having a transmission ratio Z.sub.in :Z.sub.out of approximately 2:1 formed from three segments of coaxial transmission line. Ferrite beads are provided on the outside of the coaxial transmission lines to increase their inductance.
The input and output impedance of Barabas's transformer are given by Z.sub.in =Z.sub.o =(3/2) Z.sub.otr and Z.sub.out =Z.sub.o /2 =(2/3) Z.sub.otr where Z.sub.o is the characteristic impedance of the input and output lines and Z.sub.otr is the characteristic impedance of the coaxial transmission line segments. It is not possible to simultaneously match the impedance of input and output. However, for many applications, impedance transformation is more important than impedance matching. Hence, the coaxial transmission line transformer described by Barabas is useful. The coaxial line impedance transformer circuit described by Barabas is shown schematically in FIG. 1.
While Baraba's coaxial transmission line impedance transformer can give good results and can operate over a relatively broad frequency range (2.4-5000 MHz according to Barabas), the mechanical implementation taught by Barabas (see FIG. 2) is awkward and difficult to combine with other electronic functions. Further, it utilizes expensive materials and requires precision assembly. Accordingly, a need continues to exist for more convenient and compact implementations of the impedance transformer described by Barabas, particularly, implementations that are simple, inexpensive and well suited to production applications.